Electrophoresis is an analytical technique to separate and identify charged particles, ions, or molecules. It involves the imposition of an electric field to move charged species in a liquid medium. The most often studied species are bio-macromolecules, such as proteins and DNA fragments, which are usually polyelectrolytes. However, electrophoresis can be used to separate any charged materials including various cells, bacteria and viral materials. At a fixed pH and ionic strength, a given polyelectrolyte acquires a certain number of net charges. Such particles are surrounded by counter-ions and have various charges, sizes (volume and shape) which affect movement. Molecules are separated by their different mobilities under an applied electric field. The mobility variation derives from the different charge and frictional resistance characteristics of the molecules. The more charged and streamlined the molecules, the faster their movement.
When a mixture containing several molecular species is introduced into an electrophoretic separation medium and an electric field is applied, the different charged components migrate at various speeds in the system leading to the resolution of the mixture. Bands appear, depending on the mobilities of the components. The exact location (thus time of emergence of the components at the end of the medium opposite to the point of introduction) depends on the interaction of the polyelectrolytes with the surrounding medium, via the influence of pH, ionic strength, ion type and whether the medium is a buffered solution of ions, polymeric solution, or gel such as a cross-linked gel. Cross-linked gels and polymeric solutions can effect separation by size or sieving. Hence, electrophoresis can be classified into two basic types including (1) free solution and (2) gel electrophoresis. The most frequently used gel media are based on polyacrylamide (known as PAGE) and agarose gels.
The combination of free solution and gel electrophoretic separation experiments gives a plethora of information, such as the number and relative amounts of the components in a mixture. When the components are specifically identified, e.g., by antigen-antibody binding, unequivocal identification of the presence of the given component is afforded. As a consequence, electrophoresis has become the cornerstone of macromolecular analysis in biotechnology.
Traditional electrophoretic media are large in dimension, being on the order of mm. A recent development in the field of electrophoretic technology is high performance capillary electrophoresis. In accordance with this methodology the separation media are made of hollow fibers with an inner diameter of 25 to 100 microns. This separation configuration allows a high current to pass through the media in the hollow fiber. The Joule heating can be efficiently removed due to the small size of the capillary. As a result, sample analysis time is greatly reduced. However, such high performance electrophoresis still requires the use of a high voltage DC power supply (e.g., up to tens of kv) to accomplish separation. In addition, gel-filled capillaries are not commercially available and are difficult to make. The parallel idea in slab gel media involves the use of very thin slabs. Here, gel formation involves sufficient shrinkage, making the edges of the slab more stressed than the center. Hence, if the same sample is separated using both an edge and a center track, it will exhibit different resolved patterns. This is a major problem in slab gel electrophoresis. Here again, as in capillary electrophoresis, high voltage power supplies are needed and the resulting resolution (although the best available) is not entirely satisfactory.
In both capillary and slab electrophoresis, the field applied is fixed, thus passive. Although crossed field and pulsed field electrophoresis (2D electrophoresis) methods have been described, their application is limited. In all cases, the entire separation media, free solution or gel, are under the same applied field at a given time. The field may change temporally, but not spatially by design. Accordingly, the entire sample of charged particles to be separated are under the same field at any given time.
There is also an interest in being able to move small volumes in a rapid and efficient manner without electroendoosmotic mixing. By having a system whereby particular reactant solutions can be moved by electrokinetic forces, one can carry out reactions with small volumes and within a small space.